1. Field of the Invention
The present invention relates to a highly nonlinear optical fiber and an optical device using the optical fiber.
2. Related Background Art
In wavelength conversion or the like using the nonlinear optical phenomena, a highly nonlinear optical fiber such as a dispersion-shifted fiber is used as a medium to cause the nonlinear optical phenomena (e.g. Japanese Patent Application Laid-Open No. 8-95106). The development of optical fiber in such usage has been focused heretofore mainly on improvement in nonlinearity and decrease of the dispersion slope. It is also important to reduce the variation in the zero dispersion wavelength. However, the decrease of the dispersion slope leads to increase of variation in the zero dispersion wavelength in the longitudinal direction of the filter. In addition, no attention has been directed heretofore to the fourth order dispersion β4 of the fourth derivative β4 of the propagation constant β by angular frequency, which is important to improvement in the wavelength conversion bandwidth.
For example, a reference of “M. E. Marhic, et al., Optics & Photonics News (September 2004) pp. 21-25 (2004)” describes that the bandwidth in an OPA (optical parametric amplifier) is expanded by decrease of the fourth order dispersion β4 of optical fiber. Furthermore, for example, a reference of “M-C. Ho, et al., J. of Lightwave Technol. Vol. 19, No. 7, pp. 977-981 (2001)” reports wide-band OPA using the optical fiber with the fourth order dispersion β4 being −5.8×10−56 s4/m. However, there is the description “large variation of dispersion” in the section “B. Experimental Setup for OPA Gain Measurement” on page 978 in this “M-C. Ho, et al., J. of Lightwave Technol. Vol. 19, No. 7, pp. 977-981 (2001)”, and the decrease of the fourth order dispersion β4 is insufficient. A reference of “M. Gao, et al., Optics Express, Vol. 12, No. 23, pp.5603-5613 (2004)” describes execution of optimization of fiber parameters including the fourth order dispersion β4, but fails to give consideration to such phenomena as variation in the zero dispersion wavelength and coupling of orthogonal polarization mode which must be significant issues in practical fiber.
As discussed above, there were proposals on the fiber parameters from the viewpoint of use of optical fiber, but there was no study from the aspect of production of optical fiber; it was thus difficult to produce an optical fiber with the parameters as proposed. For example, a reference of “T. Okuno, et al., OFC 2004, MF21” and other references describe such known fibers as an optical fiber having the conversion bandwidth of 91.3 nm in the fiber length of 100 m and an optical fiber having the conversion bandwidth of 110 nm in the fiber length of 100 m, but they were achieved by simply shortening the optical fibers, without optimization of the dispersion parameters.
A reference of “J. Hiroishi, et al., ECOC2002 Post Deadline Papers, PD1 (2002)” describes an optical fiber with a so-called W-shape index profile including a center core part, a depressed part, and a cladding part, and shows 1.0×10−4 ps4/km (=1.0×10−55 s4/m) as a typical value of the fourth order dispersion β4. In fact, the value of the fourth order dispersion β4 can be adjusted even in the case of the W-shape index profile, but no consideration is given to the significance of the fourth order dispersion β4. The wide bandwidth is achieved by decreasing the dispersion slope to +0.013 ps/nm2/km, but the wavelength conversion bandwidth by four-wave mixing is limited to below 40 nm, presumably, because of large fluctuation in the zero dispersion wavelength in the longitudinal direction in practice.
The Inventor discovered that the fourth order dispersion β4 could be adjusted in practical optical fibers and that a wider bandwidth could be achieved actually in the wavelength conversion, OPA, etc. by decreasing the fourth order dispersion β4 and suppressing the variation in the zero dispersion wavelength in the length direction of optical fiber, thereby accomplishing the present invention.